The present invention relates to a stoichiometric B1-type tantalum nitride and a sintered body thereof and to a method of synthesizing the stoichiometric cubic or B1-type tantalum nitride by impact compression.
The tantalum nitride with a B1-type crystal structure has the excellent properties such as high hardness, stability at high temperatures and superconductivity, as compared with hexagonal tantalum nitride.
However, it is technically difficult to synthesize the compound and no attempts have been made to produce it but for by the following means:
(1) Kieffer et al. demonstrated that the B1-type tantalum nitride could be formed by heating a hexagonal tantalum nitride at 1700.degree. C. or higher under high pressure in a nitrogen gas atmosphere (Mogatshefte fur Chemie 102, 483-485 (1971)). The resulting substance was found to have lattice constant of 0.4344.about.0.4357 nm and to be a mixture with Ta.sub.2 N in the form of a powder having a small nitrogen content.
(2) Boiko et al. obtained a B1-type tantalum nitride by heating a hexagonal tantalum nitride at 1800.degree. C. or higher under the high pressure of 30-100 Kbar, confirming that the resulting B1-type tantalum nitride showed superconductivity at 6.5K (JETP Lett. 12, 70(1970)). The resulting substance was found having a lattice constant of 0.4385.+-.0.0001 nm and to be a mixture with Ta.sub.2 N in a bulk form.
(3) Matsumoto et al. obtained a B1-type tantalum nitride by heating a hexagonal tantalum nitride in an Ar-N.sub.2 mixed plasma jet and then quenching the material, confirming that the resulting B1-type tantalum nitride exhibited superconductivity at 8.3K (Common Met. 60, 147 (1978)). The substance was found to have lattice constant of 0.433 nm and to be a mixed powder of Ta.sub.2 N and hexagonal TaN.
(4) Matsumoto et al. obtained a B1-type tantalum nitride by nitriding a plate of tantalum with an Ar-N.sub.2 mixed plasma jet under the pressure reduced to 200 Torr, confirming that the resulting B1-type tantalum nitride exhibited superconductivity at 9K (Journal of Ceramic Industry Association 95 [1] 1987, 92-93)). The resulting substance was found to have a lattice constant of 0.433.about.0.434 nm and to be a thin film represented by the chemical formula of TaN.sub.0.85-0.95.
(5) Kawada et al. filed a patent application (Japanese Patent Laid Open No. 29269/1987) for a method of producing a very fine powder of B1-type tantalum nitride wherein a laser beam was irradiated to a hexagonal tantalum nitride powder in an Ar gas under the pressure of 50.about.760 Torr and immediately thereafter the atmosphere was quenched. The resulting substance was found to be a very fine powder but its chemical composition, lattice constant and the like are unknown. It appears that this method can hardly be put to use for mass production because of its low yield of B1-type tantalum nitride.
(6) Petrumiu et al. synthesized a B1-type tantalum nitride by combusting a tantalum mass itself in a nitrogen gas atmosphere under high pressure and at a high temperature (Poroshkovaya Metalluragiya, No. 3, 62(1980)). The resulting substance was found to have a lattice constant of 0.431 nm and to be a powder represented by the chemical formula of TaN.sub.1.15-1.25.
These B1-type tantalum nitrides obtained according to the prior arts have been non-stoichiometric compositions or mixtures with Ta.sub.2 N, hexagonal TaN or the like; as some are denitrified or some comprise excessive nitrogen as a part of their forced solid solution, each representing a big departure from the stoichiometric composition as it should be. In the final analysis, any stoichiometric B1-type tantalum nitrides have not as yet been obtained as a material having the industrial usefulness. The reason for this lies in the fact that B1-type tantalum nitrides are hard to stabilize at ordinary temperatures and under ordinary pressure, because they are a phase of high temperature and high pressure.